1. Field of Art
The disclosure relates generally to earth boring bits used to drill a borehole for applications including the recovery of oil, gas or minerals, mining, blast holes, water wells and construction projects. More particularly, the disclosure relates to percussion hammer drill bits. Still more particularly, the disclosure relates to percussion hammer drill bits with an erosion retarding case.
2. Background of Related Art
In percussion or hammer drilling operations, a drill bit mounted to the lower end of a drill string simultaneously rotates and impacts the earth in a cyclic fashion to crush, break, and loosen formation material. In such operations, the mechanism for penetrating the earthen formation is of an impacting nature, rather than shearing. The impacting and rotating hammer bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole created will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical percussion drilling assembly is connected to the lower end of a rotatable drill string and includes a downhole piston-cylinder assembly coupled to the hammer bit. The impact force is generated by the downhole piston-cylinder assembly and transferred to the hammer bit via a driver sub. During drilling operations, a pressurized or compressed fluid (e.g., compressed air) flows down the drill string to the percussion drilling assembly. A choke is provided to regulate the flow of the compressed fluid to the piston-cylinder assembly and the hammer bit. A fraction of the compressed fluid flows through a series of ports and passages to the piston-cylinder assembly, thereby actuating the reciprocal motion of the piston, and then is exhausted through a series of passages in the hammer bit body to the bit face. The remaining portion of the compressed fluid flows through the choke and into the series of passages in the hammer bit body to the bit face. The compressed fluid exiting the bit face serves to flush cuttings away from the bit face to the surface through the annulus between the drill string and the borehole sidewall.
The hammer bit body may be generally described as cylindrical in shape and includes a radially outer skirt surface aligned with or slightly recessed from the borehole sidewall and a bottomhole facing cutting face. The earth disintegrating action of the hammer bit is enhanced by providing a plurality of cutting elements that extend from the cutting face of the bit for engaging and breaking up the formation. To promote efficient penetration by the hammer bit, the bit is “indexed” to fresh earthen formations for each subsequent impact. Indexing is achieved by rotating the hammer bit a slight amount between each impact of the bit with the earth. During drilling operations with the hammer bit, the borehole is formed as the impact and indexing of the drill bit, and thus cutting elements, break off chips of formation material which are continuously cleared from the bit path by pressurized air pumped downwardly through ports in the face of the bit.
In oil and gas drilling, the cost of drilling a borehole is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit, or other component of the percussion drilling assembly, must be changed before reaching the targeted formation. Each time a drilling assembly component is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new component installed, the drilling assembly must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense.
Some conventional percussion drilling assemblies include a top sub coupled to the lower end of a drill string, a driver sub, a tubular case axially disposed between the top sub and driver sub, a hammer bit received by the driver sub, and a bit retainer engaging the lower end of the driver sub. During drilling operations, chips of formation material cleared from the bit path by pressurized air pumped downwardly through ports in the face of the bit are carried by the pressurized air from the borehole upward through the annulus between the drilling assembly and the borehole sidewall to the surface. Due to geometric differences between the outer surfaces of the drill bit, bit retainer and case, localized turbulent fluid flow develops adjacent the lower end of the case. The combined effect of the turbulent fluid flow and the chips of abrasive formation material suspended therein causes erosion of the case outer surface in this portion. Over time, erosion of the case weakens the case and hastens the need for replacement of the case. As described above, replacement of the case is a time consuming and costly procedure.
Accordingly, there is a need for a case for percussion drilling assemblies and hammer bits that prolongs the service life of the drilling assembly and thus postpones the need for its replacement.